The site at Waun Ddu is a round-angled and ramparted square enclosure 30m across internally. There is a subcircular mound, 30m in diameter and 1.3m high with a summit area 15m in diameter, set over its northern angle.

The site is located on a slight knoll 50m to the west of the Roman road between Brecon to Llandovery on ground falling away to the north-west. It is likely to be a Roman fortlet and the mound a viewing feature to overcome a rise in ground level to the south-east that obscures the view along the road. An alternative suggestion for the mound is that it is the site of a small medieval castle, but its small size and remote location makes this unlikely.

Sources: Jones 1970 (BBCS 23.1 (1968)), 100-103;

J.Wiles, RCAHMW, 1 October 2004